A known gas sensor for detecting the concentration of a specific gas such as oxygen includes at least one cell composed of a solid electrolyte member and a pair of electrodes. When the temperature of the solid electrolyte member rises, the solid electrolyte member becomes active, and an electromotive force is produced between the two electrodes in accordance with the difference in oxygen concentration between two atmospheres separated by the solid electrolyte member. The solid electrolyte member is heated by the heat of exhaust gas discharged from an internal combustion engine; however, in some cases, a heater is provided so as to quickly activate the solid electrolyte member. Also, since the internal resistance of the cell has a correlation with its temperature, the temperature control of the heater is performed through feedback control based on the internal resistance of the cell (see, for example, Patent Document 1). In the apparatus disclosed in Patent Document 1, electric power is supplied to the heater such that the internal resistance of the cell coincides with a target resistance, whereby the cell is maintained at a constant or fixed temperature.
Incidentally, it has been known that when the cell deteriorates because of, for example, elapse of time, the internal resistance changes. Even when the cell deteriorates, the cell can be maintained at the constant temperature by, for example, changing the target resistance in accordance with the degree of deterioration of the cell. The apparatus disclosed in Patent Document 1 utilizes the phenomenon that, when the cell deteriorates and its internal resistance changes, the electric power supplied to the heater changes. When the electric power supplied to the heater exceeds a predetermined level, the cell is determined to have deteriorated, and the target resistance is changed to a target resistance corresponding to the degree of deterioration.